This invention relates generally to adsorptive separation of triglycerides. More specifically, this invention relates to a novel process for separating a first triglyceride comprising a docosahexaenoic acid residue from at least one other triglyceride using a chromatographic separation zone having a stationary phase which comprises metal ions that are capable of coordinating with a double bond of a fatty acid residue of the first triglyceride to form a metal complex with the fatty acid residue.
Docosahexaenoic acids (xe2x80x9cDHAxe2x80x9d) are 22-carbon, naturally-occurring, unbranched fatty acids containing 6 carbonxe2x80x94carbon double bonds. It is well known that many triglycerides comprising a DHA residue (and particularly triglycerides comprising two DHA residues) have beneficial nutritional and pharmaceutical properties. For example, such compounds may be used to treat cardiovascular and inflammatory diseases. They also may be added to infant formula to promote the development of brain and retina functions.
As used herein, a xe2x80x9ctriglyceridexe2x80x9d is an ester of three fatty acids and glycerol, and has the general chemical formula: CH2(OOCR1)CH(OOCR2)CH2(OOCR3), wherein OOCR1, OOCR2, and OOCR3 are each fatty acid residues. Each residue may be saturated (i.e., all the bonds between the carbons are single bonds) or unsaturated (i.e., the residue contains one or more carbonxe2x80x94carbon double or triple bonds), and the double bonds may each have a cis or trans configuration. To illustrate, a triglyceride having two 4, 7, 10, 13, 16, 19-DHA residues (i.e., a fatty acid residue containing 22 carbons and 6 carbonxe2x80x94carbon double bonds between the 4th and 5th, 7th and 8th, 10th and 11th, 13th and 14th, 16th and 17th, and 19th and 20th carbons, counting from the carbonyl group of the residue, the carbon of the carbonyl group being the first carbon counted) and one palmitic acid residue (i.e., a saturated fatty acid residue comprising 16 carbons) may have the following Formula (I) (without taking into consideration whether the double bonds have cis or trans configurations): 
Likewise, a triglyceride having a 4, 7, 10, 13, 16, 19-DHA residue, a 4, 7, 10, 13, 16-docosapentaenoic acid (xe2x80x9cDPAxe2x80x9d) residue (i.e., a fatty acid residue containing 22 carbons and 5 carbonxe2x80x94carbon double bonds between the 4th and 5th, 7th and 8th, 10th and 11th, 13th and 14th, and 16th and 17th carbons, counting from the carbonyl group of the residue, the carbon of the carbonyl group being the first carbon counted), and a palmitic acid residue may have the following formula (II) (without taking into consideration whether the double bonds have cis or trans configurations): 
Unsaturated fatty acid residues of triglycerides are sometimes identified in the literature by an omega (xe2x80x9cxcfx89xe2x80x9d) number. This nomenclature is used herein. The omega number denominates the position of the first double bond, when counting from the terminal methyl group of the fatty acid residue. For example, in Formula (II), the DHA residue is an xcfx89-3 fatty acid residue, and the DPA residue is an xcfx89-6 fatty acid residue. Often, fatty acid residues (and particularly the DHA residues) having the most beneficial cardiovascular effects are xcfx89-3 fatty acid residues, although other fatty acid residues (such as arachidonic acid and xcex3-linolenic acid, which are both xcfx89-6 fatty acid residues) also have proven to be beneficial as well.
Sources of triglycerides containing at least one DHA residue include, but are not limited to, modified vegetable oils, marine animal oils (e.g., seal and whale blubber), fish oils (e.g., menhaden oil, salmon oil, mackerel oil, cod oil, herring oil, sardine oil, capelin oil, and tuna oil), marine algae, and human milk. Such sources, however, typically contain many other categories of compounds. These compounds include various other triglycerides containing a wide variety of fatty acid residues (e.g., the xcfx89-6 DPA residue in Formula (II)) having varying degrees of nutritional or pharmaceutical value. It is therefore often desirable to separate a triglyceride containing at least one DHA residue (especially a triglyceride containing 2 DHA residues) from other compounds in the source before the triglyceride is used for nutritional or pharmaceutical purposes.
Numerous methods have been used alone or in combination to isolate (or at least concentrate) and recover specific fatty acids and their derivatives (i.e., esters, amides, triglycerides, etc.) from various naturally occurring sources. These processes include fractional crystallization at low temperatures, molecular distillation, urea adduct crystallization, extraction with metal salt solutions, super critical fluid fractionation on countercurrent columns, and stationary bed chromatography.
A stationary bed chromatographic system may employ a non-polar stationary phase (i.e., a reverse phase chromatographic system) or a polar stationary phase (i.e., a normal phase chromatographic system). Use of a reverse phase chromatographic system is disclosed in T. Nakahara, T. Yokochi, T. Higashihara, S. Tanaka, T. Yaguchi, and D. Honda, xe2x80x9cProduction of Docosahexaenoic and Docosapentaenoic Acids by Schizochytrium sp. Isolated from Yap Islands,xe2x80x9d JAOCS, vol. 73, no. 11, pp. 1421-26 (1996). Nakahara et al. report isolating four triglycerides containing DHA residues from a marine micro algae (i.e., Schizochytrium sp.) by a reverse-phase, high-performance liquid chromatographic process using an acetone/acetonitrile mobile phase and an octadecylsilane (xe2x80x9cODSxe2x80x9d) stationary phase. This stationary phase separates the triglycerides based on the strength of the Van der Waals forces between the stationary phase and the fatty acid residues of the triglycerides. Such a stationary phase tends to be costly relative to other conventional stationary phases (e.g., silica gel) used in normal-phase chromatography.
Others have disclosed using high performance liquid chromatography which employs a stationary phase comprising silver ions. Separation using such a column is based on the principle that a metal ion (e.g., a silver ion) will coordinate reversibly with electrons of a xcfx80 orbital of a double bond between carbon atoms of an unsaturated fatty acid residue to form a metal complex with the unsaturated fatty acid residue. The strength of the complex tends to be dependent on, for example, the chain length of the fatty acid residue, the number of carbonxe2x80x94carbon double bonds in the residue, the positions of the double bonds, and whether the double bonds have cis or trans configurations. Although this technique has been used to separate triglycerides from mixtures of triglycerides, the types of triglycerides which have been separated by this technique have been limited to date. Application of silver ion high performance liquid chromatography to separate triglycerides has been discussed in, for example, I. Elfman-Borjesson, S. V. D. Hark, and M. Harrod, xe2x80x9cGradients of n-Heptane and Acetonitrile in Silver-Ion High-Performance Liquid Chromatography Analyses of cis and trans Bonds in Lipids,xe2x80x9d JAOCS, vol. 74, no. 9, pp. 1177-80 (1997). A separate discussion of such an application may be found in P. Laakso and P. Voutilainen, xe2x80x9cAnalysis of Triacylglycerols by Silver-Ion High-Performance Liquid Chromatographyxe2x80x94Atmospheric Pressure Chemical Ionization Mass Spectrometry,xe2x80x9d Lipids, vol. 31, no. 12, pp. 1311-21 (1996). An additional separate discussion of such an application may be found in G. Dobson, W. W. Christie, and B. Nikolova-Damyanova, xe2x80x9cReview: Silver Ion Chromatography of Lipids and Fatty Acids,xe2x80x9d J. Chrom. B, vol. 671, pp. 197-222 (1995). A further separate discussion of such an application may be found in B. Nikolova-Damyanova, xe2x80x9cSilver Ion Chromatography and Lipids,xe2x80x9d Advances in Lipid Methodologyxe2x80x94One, Ch. 6, pp. 182-237 (W. W. Christie, ed., The Oily Press, Ayr, Scotland, 1992).
This invention provides for a reliable and cost-effective process that may be used to separate a triglyceride comprising at least one DHA residue (and particularly triglycerides comprising two DHA residues) from its naturally occurring source, which typically contains at least one other triglyceride. This process overcomes the following primary difficulties: (1) the triglycerides involved here are highly complex molecules, and (2) there often is only a subtle difference (such as a difference of one double bond) in structure between the desired triglyceride and the other triglyceride(s) in the source.
Briefly, therefore, one embodiment of this invention is directed to a process for separating a first triglyceride comprising a docosahexaenoic acid residue from a second triglyceride. The process comprises introducing a feed mixture comprising the first triglyceride and the second triglyceride into a first chromatographic separation zone, and then isolating a first fraction of the feed mixture in the separation zone. The first fraction has a mass ratio of the first triglyceride to the second triglyceride that is greater than in the feed mixture. The separation zone comprises a stationary phase which comprises metal ions that are capable of coordinating with a double bond of a fatty acid residue of the first triglyceride.
In another embodiment of this invention directed to a process for separating a first triglyceride comprising a docosahexaenoic acid residue from a second triglyceride, the process comprises introducing a feed mixture comprising the first triglyceride and the second triglyceride into a chromatographic separation zone, and then isolating a fraction of the feed mixture in the separation zone. The fraction has a mass ratio of the first triglyceride to the second triglyceride that is greater than in the feed mixture. In this embodiment, the separation zone comprises a stationary phase which comprises silver, magnesium, or calcium ions.
A further embodiment of this invention is directed to a process for separating a first triglyceride comprising two xcfx89-3 docosahexaenoic acid residues and a pahnitic acid residue from a second triglyceride comprising an xcfx89-3 docosahexaenoic acid residue, an xcfx89-6 docosapentaenoic acid residue, and a palmitic acid residue. This process first comprises introducing a feed mixture comprising the first triglyceride and the second triglyceride into a high performance liquid chromatographic column comprising a stationary phase comprising (a) silica gel, and (b) silver or magnesium ions. The first triglyceride is then eluted from the column by contacting the stationary phase with an eluent to form an eluate having a mass ratio of the first triglyceride to the second triglyceride that is greater than in the feed mixture.
Other features of this invention will be in part apparent and in part pointed out hereinafter.
In accordance with the present invention, a novel and useful process has been developed for the separation and recovery of a desired triglyceride comprising a DHA residue (and particularly a triglyceride comprising two DHA residues) from a mixture containing at least one other triglyceride. In a particularly preferred embodiment, this process comprises separating the triglycerides in a high performance chromatographic separation zone having a stationary phase which comprises metal ions that can coordinate with at least one double bond of a fatty acid residue of the desired triglyceride to form a metal complex with the fatty acid residue. This invention is especially useful for separating triglycerides from naturally occurring sources of triglycerides such as marine animal oils (e.g., seal and whale blubber), fish oils (e.g., menhaden oil, salmon oil, mackerel oil, cod oil, herring oil, sardine oil, capelin oil, and tuna oil), marine algae, and human milk. This method has been found to be particularly useful for separating triglycerides from the oil of marine micro algae (e.g., golden marine micro algae).
The triglycerides that may be separated and recovered using the method of the instant invention generally comprise at least one DHA residue, and preferably an xcfx89-3 DHA residue. This DHA residue may be located at any of the three positions on the glycerol backbone (i.e., the residue may be OOCR1, OOCR2, or OOCR3 in the general formula CH2(OOCR1)CH(OOCR2)CH2(OOCR3)). It has been found that this method is particularly useful for separating and recovering triglycerides having two DHA residues (e.g., Formula (I) above) located in any two of the three positions on the glycerol backbone. This method is useful even if the starting mixture contains other triglycerides which comprise a DHA residue. In fact, it has been found in accordance with this invention that this method is particularly useful for separating and recovering a triglyceride having two DHA residues and a palnitic acid residue (e.g., a triglyceride having the structure of Formula (I) above) from a mixture also containing a triglyceride having a DHA residue, a DPA residue, and a palmitic acid residue (e.g., a triglyceride having the structure of Formula (II) above). And, if desired, this method may further be used to separate and recover a second triglyceride (e.g., a triglyceride having the structure of Formula (II) above) from the mixture.
Typically, unlike ion exchange column separation, the mechanism of separation here is not based on ion exchange equilibria. Instead, the separation usually is based, at least in part, on metal ions in the column coordinating reversibly with electrons of a xcfx80 orbital of a double bond between carbon atoms of an unsaturated fatty acid residue of the desired triglyceride to form a metal complex with the unsaturated fatty acid residue. This complex is of the charge-transfer type; the unsaturated fatty acid residue acts as an electron donor and the silver ion acts as an electron acceptor. Thus, essentially all the metal ions typically remain in the separation zone throughout the separation.
The structure of the chromatographic separation zone used in accordance with this invention may vary widely. The separation zone may, for example, comprise a packed column or an open tubular column. In a particularly preferred embodiment, the separation zone comprises a cylindrical column packed with a porous material (i.e., the stationary phase). Because cylindrical packed columns are presently the most preferred structure for conducting the separation, most of the following discussion focuses on embodiments which use a cylindrical packed column. It should be recognized, however, that this invention is not limited to the use of cylindrical packed column separation zones. It should further be recognized that this invention may be conducted in a batch-wise or continuous manner.
The diameter and length of the column may vary widely. For example, columns having diameters as narrow as 2 mm may be used, particularly for analytical purposes. On the other hand, commercial columns may have diameters ranging from about 5 to about 60 cm or more. Preferably, the ratio of the length to the diameter is from about 5:1 to about 125:1, more preferably from about 10:1 to about 40:1, and most preferably from about 10:1 to about 25:1.
Typically, the stationary phase is in the form of particles (sometimes referred to as the xe2x80x9cpacking materialxe2x80x9d). The preferred size of these particles may vary, depending on the diameter and length of the column, as well as the shape of the particles. Smaller packing sizes normally produce better separation; however, smaller packing sizes also generally create higher pressure drop and back pressure. Thus, the particles preferably are as small as possible without creating more back pressure than can be tolerated by the structure of the chromatographic zone. In general, the preferred size of the packing ranges from about 1 to about 1000 xcexcm in their largest dimension. For example, in one embodiment of this invention, the column has an inside diameter of from about 2 mm to about 10 mm, and the size of the particles is preferably from about 3 to about 20 xcexcm in their largest dimension. More preferably, the size of the particles in such a column is from about 3 to about 10 xcexcm in the largest dimension, and most preferably about 5 xcexcm in their largest dimension. In larger columns typically used on a commercial scale (i.e., columns having diameters of from about 5 to about 60 cm), the size of the particles is preferably from about 60 to about 200 xcexcm in their largest dimension.
The particles also preferably are regularly shaped to produce uniformity throughout the column. Typically, it is preferred that the particles be spherical because spherical particles tend to produce a tighter pack than particles having other shapes. In some situations, however, irregularly shaped particles are preferred to decrease back pressure or provide a greater surface area at a given back pressure.
The stationary phase may comprise a wide variety of materials. Preferably, the stationary phase comprises a support and metal ions. The metal ions preferably are affixed to the support so that they do not elute from the column during the separation process. It is preferred that the metal ions be capable of coordinating with a double bond of a fatty acid residue of the desired triglyceride to form a metal complex with the fatty acid residue, i.e., the metal ion preferably coordinates reversibly with electrons in a xcfx80 orbital of a double bond between carbon atoms of an unsaturated fatty acid residue of the desired triglyceride to form a metal complex with the unsaturated fatty acid residue. Examples of suitable metal ions include metal ions selected from the group consisting of silver, magnesium, and calcium ions. In a particularly preferred embodiment, the metal ions comprise silver ions. In another particularly preferred embodiment, the metal ions comprise magnesium ions. It should be noted that in some instances, it may be desirable to combine different metal ions in the stationary phase, particularly to achieve a desired selectivity.
The type of support in the stationary phase may vary widely. The support may, for example, comprise borax or a cross-linked polystyrene. In a particularly preferred embodiment, the support is silica based. For example, the support may comprise a silicate. Suitable silicate supports include, for example, zeolite, calcium silicate, and magnesium silicate (e.g., Mg3(Si4O10)(OH)2 (xe2x80x9ctalcxe2x80x9d) and FLORISIL(copyright) (Sigma Chemical, St. Louis, Mo.)). In a more preferred embodiment, the support comprises a silica gel. The metal ions may be bound to the silica gel, for example, by mixing the silica gel with a solution containing a salt of the metal ions. This method is well known in the art. See, e.g., U.S. Pat. No. 5,672,726 (binding silver ions to silica gel by mixing the silica gel with ethanol which contains 10 g of dissolved silver nitrate per 100 g of silica gel), which is incorporated herein by reference. In the most preferred embodiment, the support is a silica gel having ligands on its surface. An example of a suitable commercially available column containing a silica gel with ligands on its surface is the ChromSpher 5 Lipids HPLC Column from Chrompack (Raritan, N.J.). This particular column reportedly uses silver-modified, spherical silica gel. The ligands on the surface of the gel are sulfonic acid residues bound to silver ions. In one embodiment, the silica gel is saturated with the metal ions.
The separation zone also comprises a mobile phase (sometimes referred to as the xe2x80x9celuentxe2x80x9d). The mobile phase preferably comprises a nonpolar solvent. Generally, it is preferred that this solvent be an unsubstituted hydrocarbon. This hydrocarbon may be a straight chain, branched, or cyclic. Examples of particularly suitable hydrocarbons include pentane, hexane, cyclohexane, isohexane, heptane, and octane.
The mobile phase may also comprise a polar solvent, such as a nitrile (e.g., acetonitrile and propionitrile), ester (e.g., ethyl acetate), ketone (e.g., acetone, methyl ethyl ketone, and methyl isoamyl ketone), dichloromethane, or chloroform. The polar solvent typically makes up no greater than about 5% by volume of the mobile phase, and more preferably from about 0.1 to about 2% by volume of the mobile phase. The purpose of the polar compound is to enhance the reversibility of the complexing between the metal ions in the column and the fatty acid residues. This, in turn, reduces (a) the retention time of the triglycerides in the separation zone, and (b) the amount of eluent needed. The polar solvent preferably does not dissolve the stationary phase or interfere with the technique used to detect the peaks in the eluate emerging from the column. It is particularly preferable, for example, to use acetonitrile as the polar component of the mobile phase when the separation is (a) conducted using a stationary phase comprising a silica gel and metal ions, and (b) ultraviolet detection is used to analyze the eluate. Water, alcohols, and amines, on the other hand, preferably are avoided when using such a system.
The separation may be isocratic (i.e., the concentration of the polar component in the mobile phase during the separation is essentially constant along the entire length of the column). Alternatively, the concentration of the polar solvent in the mobile phase entering the column may vary with time during the separation, resulting in the subsequent development of a gradient in polar solvent concentration (a) versus distance along the column, and (b) versus time at any point in the column. Such a gradient tends to narrow the range of time over which a particular triglyceride emerges from the column in the eluate, thereby increasing the concentration of the triglyceride when it emerges from the column as an eluate. If a gradient is used, the degree of change in concentration of the polar component over time preferably is constant, and the concentration of the polar component in the eluent introduced into the column preferably ranges from about 0.1% by volume near the beginning of the separation to about 5% by volume near the end of the separation. For example, when acetonitrile is used as the polar component and a concentration gradient of the acetonitrile is desired, it is preferred that the acetonitrile concentration range from about 0.1% by volume near the beginning of the separation to about 2% by volume near the end of the separation, and the preferred rate of change in acetonitrile concentration is from about 0.3 to about 5.7% by volume per hour, and more preferably from about 1 to about 3.8% by volume per hour. Generally, better resolution may be achieved using a slower rate of change in the concentration of the polar component. A slower rate of change, however, also spreads out the peaks and therefore produces a more diluted product.
The column preferably is equilibrated before conducting the separation. Equilibration typically may be achieved by passing the mobile phase alone (i.e., the nonpolar and polar solvents alone, without the triglyceride feed material) through the column for a period of time, preferably from about 15 to about 120 minutes. During equilibration, the mobile phase preferably is passed through the column at the rate used during the separation. For most columns, equilibrium may be achieved by passing from about 5 to about 20 column volumes of the mobile phase through a wet column. For example, it has been found in accordance with this invention that when using two wet Chrompack ChromSpher 5 Lipids HPLC Columns in series having 2 mm inside diameters and 250 mm lengths, the mobile phase preferably is passed through the column for about 120 minutes at 0.2 cm3/min. And, for a wet ChromSpher 5 Lipids HPLC Column having a 10 mm inside diameter and a 250 mm length, the mobile phase preferably is passed through the column for about 120 minutes at 3 cm3/min.
Before the triglyceride feed material is introduced into the separation zone, it preferably is first dissolved in a solvent. It is particularly preferable to dissolve the triglyceride feed material in the same nonpolar solvent as the nonpolar solvent used in the mobile phase. Thus, for example, if the mobile phase contains cyclohexane and acetonitrile, the triglyceride feed material preferably is dissolved in cyclohexane. Generally, it is preferred to dissolve the triglyceride feed material in sufficient solvent so that the concentration of the triglyceride feed material in the solvent is from about 1 to about 10 mg/ml.
Preferably after the triglyceride feed material has been dissolved in the solvent and the separation zone has been equilibrated, the triglyceride feed mixture (i.e., the solvent and the triglyceride feed material) is introduced into a mobile phase feed stream, which is subsequently introduced into the chromatographic separation zone. The preferred loading of the triglyceride feed material may vary, depending on, for example, the column size and desired degree of separation. Generally, for a column having a 10 mm inside diameter, preferably from about 0.1 mg to about 100 mg of triglyceride feed material is loaded, and most preferably about 10 mg of triglyceride feed material is loaded. Doubling the column diameter typically enables roughly four times as much triglyceride feed material to be loaded into the column.
Typically, the separation time is from about 20 minutes to about 6 hours. More preferably, the separation time is from about 20 minutes to about 2 hours. Like the preferred loading, the preferred flow rate of the mobile phase may vary, depending on, for example, the size of the column, and the desired degree of separation. It also varies, for example, on the amount of back pressure that the equipment can tolerate. Using the teachings of this specification and the general knowledge of the art, those of ordinary skill in the art can routinely determine the proper loading and flow rate.
Flow resistance of the column usually requires that the mobile phase feed stream be introduced into the column under pressure. Typically, the mobile phase stream is introduced via a moderate-pressure or high-pressure pump. This causes the mobile phase stream containing the triglyceride feed mixture to percolate through the packing and ultimately exit the column as a series of eluates. The pressure used during the separation generally does not affect the separation. The pressure should be sufficient to impart the desired flow rate of the mobile phase through the column, but preferably is not so great as to compromise the integrity of the column structure. For most columns, the preferred pressure is from about 100 to about 6000 psi, and more preferably from about 500 to about 3000 psi.
The temperature during the separation generally is not critical. It is preferred, however, to avoid temperatures which cause the mobile phase to boil or cause the triglycerides to degrade. Typically, it is preferred that the temperature be from about 0 to about 60xc2x0 C., and more preferably from about 20 to about 25xc2x0 C.
As an eluate emerges from the column, it may be analyzed using various quantitative or qualitative analytical devices known in the art, such as refractometers, polarimeters, and detectors (e.g., ultraviolet detectors). For example, an analysis of the eluate may be conducted using an evaporative light scattering detector (e.g., model # 750/14, Applied Chromatography Systems Ltd., Macclesfield, Cheshire, England). In a particularly preferred embodiment, the eluate is analyzed using an ultraviolet detector, such as Hewlett-Packard""s Diode-Array Ultraviolet Detector (Hewlett-Packard, Palo Alto, Calif.) set at 205 nm. Using such an analysis, the eluate having the desired triglyceride composition may be isolated and recovered from the column.
In one embodiment of this invention, a second eluate also is collected to isolate and recover a second triglyceride. This triglyceride may itself be desirable, e.g., for nutritional or medical reasons. Alternatively, the second triglyceride may subsequently be hydrolyzed (i.e., the fatty acid residues of the triglycerides may be cleaved from the glycerol backbone of the triglyceride to form the corresponding free fatty acids) or transesterified (i.e., the fatty acid residues of the triglyceride may be cleaved from the glycerol backbone of the triglyceride and then used to form esters of the fatty acid residues).
Hydrolysis of the second triglyceride may be carried out by, for example, acid-catalyzed or base-catalyzed hydrolysis. In one embodiment of this invention, hydrolysis of the second triglyceride is carried out using the acid-catalyzed process comprising incubating the triglyceride at from about 20 to about 150xc2x0 C. (most preferably, at about 100xc2x0 C.) in a mixture containing water and a mineral acid (e.g., a mixture of water and HCl having an HCl concentration of from about 4 to about 10% by weight). The triglyceride concentration in the acid/water mixture preferably is from about 0.1 to about 10% by weight, and most preferably about 1% by weight. To perform the hydrolysis using a base, a base catalyst (e.g., KOH) is used instead of an acid catalyst. Regardless of whether the catalyst is an acid or a base, the hydrolysis preferably is performed in a non-oxidizing atmosphere (i.e., the atmosphere comprises non-oxidizing gases such as N2 and/or a noble gas, and preferably consists essentially of no oxidant).
In a particularly preferred embodiment, the second triglyceride is transesterified to form alkyl esters of the fatty acid residues. Transesterification, in general, is well known in the art. See, e.g., W. W. Christie, xe2x80x9cPreparation of Ester Derivatives of Fatty Acids for Chromatographic Analysis,xe2x80x9d Advances in Lipid Methodologyxe2x80x94Volume Two, Ch. 2, pp. 70-82 (W. W. Christie, ed., The Oily Press, Dundee, United Kingdom, 1993) (incorporated herein by reference). The alkyl esters formed by the transesterification preferably are lower alkyl esters of the fatty acid residues, and more preferably methyl esters or ethyl esters of the fatty acid residues, i.e., the alkyl esters have the Formula (III): 
wherein R1 preferably is a hydrocarbyl containing from 1 to 6 carbons, and more preferably from 1 to 2 carbons; and R2 is the straight chain hydrocarbyl of the fatty acid. For example, the methyl ester of 4, 7, 10, 13, 16, 19-DHA has the Formula (IV) (without taking into consideration whether the double bonds are cis or trans): 
Likewise, the ethyl ester of 4, 7, 10, 13, 16, 19-DHA has the Formula (V) (without taking into consideration whether the double bonds are cis or trans): 
Ethyl esters are generally most preferred because they typically are less toxic when eaten. For example, ethyl esters typically hydrolyze to form ethanol when digested, while methyl esters typically hydrolyze to form methanol when digested.
In one embodiment of this invention, th e triglyceride is transesterified to form the alkyl esters of the fatty acid residues by acid-catalyzed transesterification wherein the triglyceride is transesterified by an alcohol in the presence of an acidic catalyst. Acid-catalyzed transesterification may be carried out, for example, by incubating a triglyceride at from about 20 to about 150xc2x0 C. (most preferably, at about 100xc2x0 C.) in a mixture containing the alcohol and a mineral acid (e.g., HCl), preferably under a non-oxidizing atmosphere and in the absence of water (i.e., the alcohol/acid mixture preferably consists essentially of no water). In one embodiment, the triglyceride/acid/alcohol mixture is refluxed for at least about 2 hours. In another embodiment, the triglyceride/acid/alcohol mixture is maintained at from about 20 to about 50xc2x0 C. overnight. The alcohol has the formula R1-OH (wherein R1 is defined above for Formula III), and is selected to form the desired fatty acid ester. For example, methanol may be used to form methyl esters, and ethanol may be used to form ethyl esters. Because acid-catalyzed transesterification is typically reversible, the alcohol preferably is present in a large excess so that the reaction proceeds essentially to completion. Preferably, the triglyceride concentration in the alcohol/acid mixture is from about 0.1 to about 10% by weight, and most preferably about 1% by weight. In most cases, the acid preferably is HCl. If the acid is HCl, the concentration of HCl in the alcohol/HCl mixture preferably is from about 4 to about 10% by weight. Such a mixture may be prepared by various methods known in the art, such as bubbling dry gaseous hydrogen chloride into dry ethanol. Although HCl is most preferred, other acids may alternatively be used. One such acid is or H2SO4, which typically is used at a concentration of from about 0.5 to about 5% by weight in the alcohol. It should be noted, however, that because H2SO4 is a strong oxidizing agent, it preferably is not used with long reflux times (i.e., greater than about 6 hours), at high concentrations (i.e., greater than about 5% by weight), or at high temperatures (i.e., greater than 150xc2x0 C). Another example of a suitable acid is boron trifluoride, which preferably is used at a concentration of from about 1 to about 20% by weight in the alcohol. Boron trifluoride, however, is less preferred than HCl because boron trifluoride has a greater tendency to produce undesirable byproducts.
A triglyceride alternatively may be transesterified by, for example, base-catalyzed transesterification wherein the triglyceride is transesterified by an alcohol in the presence of a basic catalyst. In this instance, the transesterifying agent may, for example, be sodium or potassium methoxide (preferably from about 0.5 to about 2 M) in anhydrous ethanol. Other bases also may be used, although bases (e.g., KOH) that react in the mixture to form water are less preferred because water tends to cause the fatty acid esters to hydrolyze to form free fatty acids.
The separation of the free fatty acids or alkyl esters resulting from the hydrolysis or transesterification of the second triglyceride may be conducted using, for example, a separation zone comprising a stationary phase which comprises metal ions that are capable of coordinating with a carbonxe2x80x94carbon double bond of the desired fatty acid or ester. In one preferred embodiment, the mixture containing the free fatty acids or alkyl esters is introduced into the same separation zone used initially to isolate the triglycerides, either while the triglycerides are being separated or afterwards. The mobile phase preferably comprises a nonpolar solvent (most preferably an unsubstituted hydrocarbon, such as pentane, hexane, cyclohexane, isohexane, heptane, or octane) and a polar solvent (such as acetonitrile, ethyl acetate, dichloromethane, chloroform, or acetone). The polar solvent preferably makes up no greater than about 5% by volume of the mobile phase (more preferably from about 0.1 to about 2% by volume of the mobile phase), and does not dissolve the stationary phase or interfere with the technique used to detect the peaks in the eluate emerging from the column. The separation may be isocratic or use a mobile phase containing a polar solvent concentration gradient. The preferred loading, flow rate of the mobile phase, pressure, and temperature generally fall within the same ranges as those for triglyceride separation using the same separation zone.
Other methods for separating many free fatty acids and lower alkyl esters of fatty acids from mixtures of fatty acids and lower alkyl esters of fatty acids using silver ion high performance liquid chromatography are well known in the art. A discussion showing an example of such a method may be found, for example, in I. Elfman-Borjesson, S. V. D. Hark, and M. Harrod, xe2x80x9cGradients of n-Heptane and Acetonitrile in Silver-Ion High-Performance Liquid Chromatography Analyses of cis and trans Bonds in Lipids,xe2x80x9d JAOCS, vol. 74, no. 9, pp. 1177-80 (1997) (incorporated herein by reference) (showing the separation of the ethyl ester of DHA from a 395 xcexcg sample of fish oil that also contains the ethyl ester of eicosapentaenoic acid using a 4.6xc3x97250 mm Chrompack ChromSpher Lipids silver ion high performance liquid chromatography column, a heptane/acetonitrile mobile phase, an acetonitrile gradient of 0.1 to 0.3% by volume in the mobile phase, a mobile phase flow rate of 3 ml/min, and a separation temperature of 23 to 25xc2x0 C). A separate general discussion of such separations may be found, for example, in B. Nikolova-Damyanova, xe2x80x9cSilver Ion Chromatography and Lipids,xe2x80x9d Advances in Lipid Methodologyxe2x80x94One, Ch. 6, pp. 182-237 (W. W. Christie, ed., The Oily Press, Ayr, Scotland, 1992) (incorporated herein by reference). A further separate general discussion may be. found, for example, in G. Dobson, W. W. Christie, and B. Nikolova-Damyanova, xe2x80x9cReview: Silver Ion Chromatography of Lipids and Fatty Acids,xe2x80x9d J. Chrom. B, vol. 671, pp.197-222 (1995) (incorporated herein by reference).
Once the desired free fatty acid or alkyl ester has been isolated and recovered, the free fatty acid or alkyl ester itself may be used directly (e.g., for medical or nutritional purposes). Alternatively, if the isolated compound is a free fatty acid, the free fatty acid may be esterified, for example, to an alkyl ester. If on the other hand, the isolated compound is an alkyl ester, the alkyl ester may, for example, be hydrolyzed to form the corresponding free fatty acid or transesterified to form a different ester. This embodiment is particularly useful for recovering DHA residues from triglycerides in instances where the triglycerides themselves are less desirable due to the other fatty acid residues present in the triglycerides.
Unless otherwise stated, the term xe2x80x9chydrocarbylxe2x80x9d is defined as a radical consisting exclusively of carbon and hydrogen. The hydrocarbyl may be branched or unbranched, may be saturated or unsaturated, and may comprise a ring structure.